The invention relates to a method for inspecting fast cyclic processes by high frame rate image recording means (high speed video means/high speed camera means).
Such fast cyclic processes occur for instance in production and packaging processes involving several operation sequences (cycles)/minute and/or many short repetitive movements performed in each operation sequence.
During such fast sequences of operations different failures may take place. Such failures can result in part of the production being improper for use or sales and/or require the production process to be interrupted, or can even cause the production to be blocked.
This can involve severe economic losses.
On the other hand a process cycle can also operate in a sub-optimal manner without resulting in noticeable failures but with some loss of time in each cycle which added up for large production volumes can involve a considerable loss in comparison to an optimal situation.
One approach for analysing the defaults and/or optimising a process cycle (other than just a trial and error setting of the operation parameters of the machine) can reside in analysing slow motion images of the process cycle or part of the process cycle. This may be achieved with camera-systems recording the operation sequences at a high “frame rate”.
Conventional, known, video systems are not suitable for that purpose as the frame rate of the image recording is totally insufficient, whereas conventional, known, high speed camera systems are prohibitively expensive and do not permit an “on-line”, “real time” analysis of the images, i.e. the images can not be reproduced directly on a LCD-screen during the actual inspection.
Conventional high speed camera systems are furthermore often much too large and heavy to be easily used for this purpose, whereas ultra light versions are extremely expensive and anyway still involve the draw back of not allowing “real time” analysis of failures.
Specific attempts have on the other hand been made to adapt known video systems and/or known high speed camera systems to high speed imaging methods for inspection purposes, but such attempts involved rather complex optical or electronic constructions which did not lead to satisfactory practical results.
Thus for instance:
a high speed image pickup camera according to JP 1991.0204670, involving a high speed image input circuit with a high speed output signal therefrom which is submitted to object discrimination and signal conversion for standard video systems;
a high speed imaging method according to JP 2004.254073, involving an imaging lens with a section for splitting the optical path; and
high speed imaging for repetitive processes according to DE 4 429 966, involving a mechanically coupled optical switching system and a laser based sensing unit generating trigger signals feeding a synchronising module, designed to produce half frame signals.
A different approach for providing a motion analysis tool for viewing fast repetitive movements is disclosed in EP 0 351 558. This disclosure proposes the use of a conventional video camera together with strobe illumination of the moving object. A control system provides for capturing a series of video images over a number of cycles, where only one image is captured in a cycle and each successive image is advanced a predetermined step of time in successive cycles so that when the series of images is displayed a slow motion view of the object is provided while the object moves in repetitive motion. The analysis tool thus provides a window wherein the full cycle or only a selected portion of the cycle can be repeatedly viewed. The predetermined steps of time can be varied to increase the resolution.
The inspection method according to this document thus essentially involves the principle of recording successive images (of successive cycles) of the cyclic process, and synchronised slow motion representation of said recorded successive images during the total cycle time of the cyclic process or a multiple thereof.
In this method the slow motion representation thus consists of a recomposed sequence of successive images taken from different cycles, which in certain circumstances constitutes a drawback of the method.